a) Field of the Invention
This invention relates to an internal combustion engine mounted on a vehicle, and particularly to a control system for a lean-burn internal combustion engine which performs a lean-burn operation at an air/fuel ratio leaner than a stoichiometric air/fuel ratio under predetermined operation conditions.
b) Description of the Related Art
Lean-burn internal combustion engine (i.e., so-called lean-burn engines) have been provided in recent years, which perform a lean-burn operation at an air/fuel ratio leaner than a stoichiometric air/fuel ratio under predetermined operation conditions.
Even with such lean-burn engines, a reduction in the acceleration performance as well as a deterioration in exhaust gas may take place when a lean-burn operation is conducted upon acceleration. As is disclosed, for example, in Japanese Patent Application Laid-Open (Kokai) No. HEI 1-29642, it has therefore been proposed to perform control in such a way that upon acceleration, a lean-burn engine is operated by setting the air/fuel ratio at a stoichiometric or richer air/fuel ratio but upon ending of the acceleration, the air/fuel ratio is set back to a lean air/fuel ratio and the lean-burn engine is then operated at the lean air/fuel ratio.
For the above control, it is necessary to determine if the engine is in acceleration. This determination can be effected, for example, by a deviation in the throttle position. Namely, the engine can be determined to be in an accelerated state provided that the deviation of the throttle position is greater than a threshold.
In such a conventional lean-burn engine, the engine undergoes a torque down when the air/fuel ratio is changed from the stoichiometric air/fuel ratio to a lean air/fuel ratio. In a low-speed range of the engine, this torque down can be reduced to a small level by conducting an air assist, that is, by opening an air bypass valve to supplement air through an air bypass passage. In a high-speed range, however, the amount of air becomes insufficient even if an air assist is conducted through the air bypass passage, whereby a torque down of a certain degree occurs when the stoichiometric air/fuel ratio is changed to the lean air/fuel ratio. When the acceleration has ended and the air/fuel ratio has returned to the lean air/fuel ratio, the driver therefore feels a reduction in the vehicle speed. The driver may hence depress an accelerator pedal to maintain the vehicle speed. This results in a greater deviation in the throttle position so that the vehicle is determined to be in an accelerated state. The air/fuel ratio is accordingly changed from the lean air/fuel ratio to the stoichiometric air/fuel ratio. No lean-burn operation is therefore feasible although the acceleration has already ended and the engine is ready for a lean-burn operation.
For example, FIGS. 8 and 9 schematically illustrate changes in air/fuel ratio, engine torque, vehicle speed and throttle opening from the time of acceleration to after the end of the acceleration. FIG. 8 shows illustrative changes at a low speed whereas FIG. 9 depicts those at a high speed.
At the low speed, namely, as is illustrated in FIG. 8, when the air/fuel ratio is changed from the stoichiometric value for acceleration to the lean value as a result of completion of the acceleration, the engine torque naturally drops. As an air assist conducted through an air bypass passage at the time of the lean-burn operation is exhibiting its effects sufficiently, the engine however undergoes a reduced torque down so that a reduction in the vehicle speed can be limited to a slight level. The driver is therefore not tempted to substantially depress the accelerator pedal shortly after the switching to the lean air/fuel ratio. Even if the deviation of the throttle position increases, this deviation does not increase to such a value as exceeding an acceleration criterion. The lean-burn state is therefore continued.
At the high speed, on the other hand, the switching of the air/fuel ratio from the stoichiometric value to the lean value upon ending of the acceleration results in a substantial down in engine torque as shown in FIG. 9, because at the high speed, the air assist is less effective, resulting in insufficient air. In this case, the vehicle speed drops significantly so that shortly after the air/fuel ratio is changed to the lean value, the driver may substantially depress the accelerator pedal so that the deviation of the throttle position is increased beyond the acceleration criterion. As a consequence, the engine is operated at the stoichiometric air/fuel ratio although the acceleration has ended and the engine is ready for operation at the lean air/fuel ratio.
In a low-speed operation, there are generally more occasions of acceleration so that it is required to promptly respond each demand for acceleration. In a high-speed operation, however, there is generally a higher probability of performing a steady-state operation featuring less changes. Unless otherwise specifically demanded by the driver, it is hence preferred to perform a lean-burn operation so that the fuel consumption can be improved.